<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 200px;
            height: 200px;
            background-color: pink;
        }
        .fk{
            background-color: red;
        }
    </style>
</head>
<body>
    <div>你看这个面又长又宽</div>
</body>
</html>
<script>
    var div=document.querySelector('div')
    div.onclick=function(){
        // div.style.fontSize='50px'
        // div.style.backgroundColor='orange'
        div.className='fk'
    }
</script>